In a typical carbonless paper form, the back side of the top sheet has a coating of microscopic capsules containing colorless chromogens or color precursor in the oil. This top sheet is called CB paper, for coated back. The bottom sheet is the receiving sheet, which is coated with a color developer. It is designated as CF paper, for coated front. The intermediate plies have color developer coating on the front sides and microcapsules coated on the back sides. These sheets are called CFB. A self-contained paper has both the color developer and microcapsules coated on the same side of the web. When the CB and CF coatings are arranged in a manner of facing each other, a typing or writing pressure from the top side of CB paper will produce images on the CF surface. Similarly, images will also appear on all CF sides in a multiform set of CB, CFB's, and CF papers. A self-contained paper will develop images in situ upon the rupture of microcapsules by a typing or writing pressure.
Two principal techniques for the production of microcapsules have been used since the microencapsulation technology was introduced in the 1950's. Numerous patents have revealed in-depth details in this field. The first method is the phase separation of film-forming materials from the continuous phase by the coacervation process. It involves the deposit of colloidal materials, such as gelatin and gum arabic, around the oil droplets, followed by hardening with formaldehyde as taught in U.S. Pat. No. 2,800,457 (1957). Many other patents also disclose that aqueous solutions of hydrophilic colloids may be coacervated by adding various substances, such as inorganic salts and oppositely charged colloids, to the solutions. However, this process has a number of disadvantages in commercial production of the coacervates. It requires careful control of the cooling temperature over a long period of time for the colloids to deposit around oil droplets and continuous adjustment of pH-value for the system. Undesired agglomeration of microcapsules usually occurred during the subsequent hardening reaction with aldehyde. The polynuclear cluster of microcapsules makes it very difficult to produce sharp images in the carbonless papers copying system.
Another method is the interfacial polymerization of two direct-reacting intermediates around minute oil droplets. Generally, one reacting intermediate is dissolved in a hydrophobic liquid and the second intermediate is present in a hydrophilic liquid. In some cases, both reacting intermediates may be included in a hydrophobic liquid phase which is emulsified into an immiscible hydrophilic liquid. Polycondensation is then promoted by catalysts or heat so that the intermediates react with each other to yield a solid product as the skin at the interface of minute oil droplets. Typical examples of such condensates are polyamide, polyester, polyurethanes, polyurea, and the like. This interfacial polymerization requires proper selection of intermediates.
Inasmuch as this invention deals with polyisocyanates which have been used in microencapsulation before, their roles may be classified as follows:
1. Re-enforcement of the primary capsule wall.
The capsule wall formed by the coacervation process is normally swollen due to the moisture sensitivity of gelatin material. This primary wall is somewhat permeable, allowing the capsule core material to escape over a period of time. Thus, polyisocyanates have been used to alleviate such deficiency for the wall of hydrophobic polymers as disclosed in U.S. Pat. No. 3,660,304, the wall formed by complex coacervation as disclosed in U.S. Pat. No. 3,897,361, and the wall formed by the phase separation of poly(ethylene-co-vinyl acetate) as disclosed in U.S. Pat. No. 3,674,704.
2. Polymerization in the oil phase.
U.S. Pat. Nos. 3,726,804 and 3,796,669 disclose the production of microcapsules by dissolving film-forming materials in an oily liquid to be encapsulated, which then polymerize by itself or react with another film-forming material to produce a water-insoluble, high molecular weight material. The temperature of the system is raised to cause the deposit of such high molecular weight product on the surface of minute oil droplets.
3. Interfacial polymerization (other than with the emulsifier).
A number of patents, such as U.S. Pat. Nos. 3,432,327, 3,577,515, 3,886,085, 3,900,669, 4,021,595, 4,046,741, 4,119,565 and 4,120,518 have revealed the formation of microcapsules by the polymerization of polyisocyanate in the oil phase and a co-reactant, other than the emulsifier or protective colloid, from the outer phase of each oil droplet. The polymerized product precipitates at the interface to form the capsule wall. The co-reactant may be selected from polyamines, polyols, polycarboxylic acids, polythiols, and epoxy compounds. Catalysts are normally needed for the interfacial polymerization. Nevertheless, it is very difficult to attain a complete polymerization because the co-reactant is either dissolved or thoroughly dispersed in the aqueous phase in the resulting coating which will be directly exposed to the users, posing health and ecological problems. For example, polyamines and epoxy compounds have been suggested as carcinogens.
4. Interfacial cross-linking of the emulsifier.
The cross-linking of the emulsifier by polyisocyanates at the interface of oil droplets has been disclosed in U.S. Pat. Nos. 3,895,074, 4,025,455, 4,107,071 and 4,138,362. Polyisocyanates in the oil phase react with the water-soluble emulsifier to produce a water-insoluble film as the capsule wall. The choice of the cross-linkable polymers, both synthetic and natural, is very critical for this method. The use of catalysts is also suggested.